There are so many occasions where you will need to invite friends and family, and some of whom maybe a lot older or younger.
A great occasion where you will see guests of mixed ages is at a wedding. On these occasions the DJ has in some ways an easier job, as everyone is generally in a good mood anyway and normally by the time of the reception is ready to party! From experience the older generation (Nan's & granddad's) tend to get up on the dance floor a bit earlier, and if this is the case the DJ should either take requests or make an intelligent decision and play songs that would they would probably dance to. So, up on the dance floor dancing away you have the older generation, however now 5 people in their twenties want to join in, but they don't like, or know the old 70's song the DJ is playing. Now the DJ has to decide what they're going to play to keep everyone on the dance floor and happy. They could take a risk and change the music to something upfront and new, however there is a risk of the older generation walking off. The safest choice for the DJ is to play a tune which is very popular amongst all age groups and relatively new, as the older and younger generation will probably know and like it. A classic like Amy Winehouse and Valerie would go down well, as it is a classic, but modern sound which everyone can enjoy. Of course one song only isn't going to keep going for long, so the DJ must think of a few more at least until another group of people gets on to the dance floor.
Another occasion where you could see a mixed age group is at a Birthday party, and the DJ has a few options to make sure everyone gets there fair share of music and entertainment. The key to successful Birthday party is to ensure the person celebrating their Birthday feels important and special and the DJ should focus a good amount of attention on them. They will be excited that the DJ has announced their Birthday to everyone and people will get excited too.
Music is one way to entertain everyone, but for me another equally good way to keep everyone happy and in harmony is to use the microphone, to communicate with the audience and create the best possible atmosphere. Introducing a bride and groom to their first dance will for sure get a cheer from the audience, or saying happy Birthday will get a cheer from the entire audience. This will create atmosphere and the mixed aged audience would probably focus less on what song is one and just enjoy the atmosphere.
Another thing the DJ should do is take requests, announcing this over the microphone to the entire audience. In this case people may not like a particular song that is being played at the time, but they could wait some time for their requested song to be played. Again, allowing more and more people to fill up the dance-floor as the night goes on.
If the DJ has a busy dance floor, they will find it easier to keep it that way, providing the song choices are good and cater for a wider audience. Playing a specific genre could entertain a couple of people on the dance-floor, however others may leave if the song is un-commercial and unknown.
Above all keeping the music commercial, whether it be old or new would normally satisfy any crowd, unless a specific genre has been requested.